Radio transmission of a signal in a given environment is subject to numerous degradations. Signals are subject to reflections and refractions due to the presence of obstacles between the emitter and the receiver. This results in the reception of multiple copies of the emitted signals by the receiver. Transmitted signals are also subject to interferences and noise due to the radio environment.
Transmission of digital data over a radio link implies typically several steps. Digital data are constituted by sequences of bits, 0 or 1, to be transmitted. First is applied to the sequence a channel coding step. Different kinds of channel coding schemes exist. They all have the goal to add some redundancy to the bit sequence to allow reconstruction of emitted bits after transmission even when some of the emitted bits are lost. The encoded bit sequence is then subject to interleaving to break the dependency of successive bits in the sequence. Bits are mapped to modulation symbols according to the modulation scheme which can be QPSK (Quadrature Phase-shift Keying), QAM (Quadrature Amplitude Modulation) or others. Symbols are then emitted over the radio link according to the adopted air interface. The air interface defines the way the radio resource is used for the transmission and how it is shared among different users. One can cite here the transmission scheme OFDM (Orthogonal Frequency Division Multiplexing) which transmits multiple data symbols in parallel using different orthogonal frequency carriers, the multiple access schemes TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access) and CDMA (Code Division Multiple Access), and the multiple antennas techniques MIMO (Multiple Input Multiple Output) and beamforming. An air interface can be any combination of these schemes as OFDMA-MIMO which combines OFDM and FDMA schemes and applies MIMO techniques. At the receiver, the signal is symmetrically analyzed. It is first demodulated, besides an equalization step is applied followed by a symbol de-mapping step, a bit de-interleaving step, a channel decoding step that gives an estimation of emitted bits. The choice of the modulation scheme and the channel coding scheme leads to the Modulation and Coding Scheme (MCS). The mechanisms considered at the emitter and receiver define what is called the physical layer of the radio transmission.
We have seen that the channel coding scheme applies to a sequence of bits. This sequence for a given channel coding scheme defines what is called a codeword which is also referred as a frame in the transmission system. The choice of a given set of physical layer mechanisms defines the physical layer mode which is mainly characterized by its transmission rate and error rate. The error rate can be expressed in term of the Bit Error Rate (BER) or Frame Error Rate (FER) which is the percentage of erroneous received frames of data through the physical layer. As an example, the choice of a robust MCS, with a small number of different symbols in the modulation scheme and a high level of redundancy in the coding scheme will lead to a low transmission rate and low FER even on a bad quality radio channel while a good quality radio channel will allow a less robust MCS leading to a greater transmission rate. To optimize the use of the radio resource during the transmission it is important to adjust the chosen physical layer mode that gives the highest transmission rate keeping an acceptable FER. The notion of acceptable FER depends on the application.
The idea of radio link adaptation and scheduling is to choose the physical layer mode and allocate the radio resources giving the highest quality of transmission below the threshold of what is defined to be the acceptable quality of transmission of a given application. The quality of transmission is relative to the FER. A function of the FER, or the FER itself, can be chosen as the indicator of the quality. The problem that arises is that it is not possible to know the FER at the receiver (respectively the transmitter) because we don't know the emitted (respectively the received) data. Therefore, there is a need to provide an estimate of the FER. The accuracy of this estimation is crucial for the efficiency of the link adaptation and scheduling mechanisms. This estimation is done by applying what is called a quality model that gives an estimation of the true FER (FERt), called FERe, from values accessible on the duration of the transmission.
In order to be able to build a good estimation of FER, we need to find out a function to compute an estimation of FER called FERe. The generic form of this function is:FERe=fe(S1, . . . , SN)  (1)
Where fe takes as entry parameters the chosen values {Sn} that are accessible on the duration of transmission and computes the estimate value FERe. With no loss of generality, the fe function can be written as a composition of two functions fm, called the mapping function, and fc, called the compression function:FERe=fe(S1, . . . , SN)=fm∘fc(S1, . . . , SN)=fm(Seff);  (2)Seff=fc(S1, . . . , SN)
Seff is called the effective measure on the transmission duration. It defines a quality metric on the transmission. fm is a correspondence table established on a simulation platform.
The basic idea leading to the computation of FERe is that it is possible to define a quality model that accurately takes into account the receiving chain. This quality model gives a way to compute FERe from a judicious choice of relevant measures {Sn}. It defines the values {Sn}, the compression function fc and the mapping function fm. This is illustrated FIG. 1. On this figure is presented a schematic view of a receiver 1.1. The received signals 1.6 are treated by the receiving chain 1.5 to decode the received data {circumflex over (d)}. The received signals 1.6 are also treated by a quality estimation model 1.2 constituted by a measurement module (MM) 1.3 that makes a set of measures {Sn} which is treated by the quality model (QM) 1.4 to build the estimate value FERe by first computing the Seff value with the fc function and then apply the fm to Seff to obtain FERe. It is clear for the man skilled in the art that once the measured values {Sn} have been obtained, the actual computation of FERe can be done anywhere in the system, on the receiver, on the emitter or even on another device of the system. The two key points of this quality estimation model is the relevance of the chosen measures {Sn} and the accuracy of the quality model relatively to the receiving algorithms used in the receiving chain and the characteristics of the physical channel.
Some quality models, including a set of measures is {Sn} and a quality metric Seff have been proposed, like the exponential effective SINR model or the generalized exponential effective SINR model in Third Generation Partnership Project (3GPP), “System-Level Evaluation of OFDM—Further Considerations,” TSG-RAN, WG1 #35, RI-031303, or “Link Performance Models for System Level Simulations of Broadband Radio Access Systems”, by K. Brueninghaus et al. in the proceedings of IEEE PIMRC conference, September 2005. A review of proposed models can be found in “On the System Level Performance of MC-CDMA Systems in the downlink” the PHD thesis of the inventor at Ecole nationale supérieure des télécommunications de Bretagne, January 2006. Another model have been proposed by the inventor in the European application EP 04293044.6 filed by the applicant on Dec. 20, 2004.
All these models have been established in the context of a perfect knowledge of the measured values {Sn} which generally depend on the propagation conditions of the transmission channel and on the interferences and noise affecting the signal. But when transposing these models to implement real receivers either the receiving chain and/or the measurement model are susceptible to be affected by the fact that this perfect knowledge of the propagation conditions and interferences and noise affecting the transmission can never be achieved. The values {Sn} are measured on the received signal, this process is prone to measurement errors and leads to estimated values {Ŝn} instead of exact values {Sn}, estimated effective measure Ŝeff and a degraded estimate of FERe called FÊRe:FÊRe=fm(Ŝeff); Ŝeff=fc(Ŝ1, . . . , ŜN)  (3)